1. Field of the Invention
The present invention relates to a tee nut having a hollow shaft portion provided with a female screw, i.e. a female thread, on its inner peripheral surface and a flange portion outwardly extending from an end of the shaft portion, and more particularly, it relates to a rivet type tee nut including a predeterminate caulked part on an end of the shaft portion opposite to the flange portion.
2. Description of the Prior Art
For example, U.S. Pat. No. 5,238,344 or 5,348,432 discloses a conventional tee nut having a thin part to be caulked on its shaft portion. FIGS. 14A and 14B show a tee nut 11 described in U.S. Pat. No. 5,238,344, which is integrally obtained by performing sheet metal working on a ferrous metal plate, for example, and includes a shaft portion 12 and a flange portion 13 outwardly extending from a first end of the shaft portion 12.
The shaft portion 12 is in the form of a hollow cylinder having a uniform outer diameter and has a predeterminate caulked part 14 on a second end opposite to the first end, while a female screw, i.e. a female thread 15a is formed on the inner peripheral surface of a female screw forming part 15 excluding the predeterminate caulked part 14. The predeterminate caulked part 14 is smaller in thickness than the portion provided with the female screw forming part 15. Thus, the shaft portion 12 can be threaded from either the first or second end thereof for forming the female screw 15a.
The flange portion 13 is provided with two pawls 16, which are formed by partially inwardly crushing the outer peripheral edge of the flange portion 13 to project toward the second end of the shaft portion 12 in opposition to each other at 180.degree.. Notches 17 having substantially semicircular sections are left on the outer peripheral edge of the flange portion 13, due to such formation of the pawls 16.
The tee nut 11 is used as shown in FIG. 16A, for example. Referring to FIG. 16A, the shaft portion 12 is first inserted in an object 19 of a resin material or wood previously provided with a through hole 18 and then the predeterminate caulked part 14 is caulked with a caulker in this state for forming a caulked part 14a on a surface of the object 19. At the same time, the pawls 16 are made to bite into the other surface of the object 19. Thus, the tee nut 11 is completely fixed to the object 19.
FIGS. 15A and 15B show a tee nut 31 described in U.S. Pat. No. 5,348,432, which is integrally obtained by performing sheet metal working on a ferrous metal plate, for example, similarly to the aforementioned tee nut 11, and includes a shaft portion 32 and a flange portion 33 outwardly extending from a first end of the shaft portion 32. The tee nut 31 has a female screw forming part 35 provided with a female screw 36 and a predeterminate caulked part 34 on the inner periphery of the shaft portion 32. This tee nut 31 is different in structure from the tee nut 11 in a point that two pairs of pawls 37, 38, 39 and 40 extending from a first end to a second end are arranged on the outer periphery of the flange portion 33 to be opposed to each other along the diametral direction of the flange portion 33, and the flange portion 33 has a substantially octagonal shape as a whole.
Such a tee nut 31, which can be automatically supplied by being smoothly moved along a feed track of a nut fixer for fixing the same to an object, is generally called a hopper feed tee nut. For example, U.K. Patent No. 1,157,734 describes an exemplary hopper feed tee nut in detail.
FIG. 15B shows the aforementioned feed track 43 with phantom lines. The feed track 43 includes a pair of guide rails 44 and 45 having C-shaped sections, which are symmetrically arranged to be opposed to each other. The guide rails 44 and 45 receive the flange portion 33 while positioning the pawls 37 to 40 therebetween, for moving the tee nut 31 along the feed track 43 in a prescribed attitude. The feed track 43 is often bent (the bent portion is not shown) for bringing the tee nut 31 into a desired attitude and aligning the shaft portion 32 with a hole provided on an object (not shown).
This tee nut 31 is used in a mode similar to that for the tee nut 11 as shown in FIG. 16B, for example. The shaft portion 32 of the tee nut 31 is inserted in a through hole 18 of an object 19 and then the predeterminate caulked part 34 is caulked with a caulker in this state for forming a caulked part 34a on one surface of the object 19. At the same time, the pawls 37 to 40 are made to bite into the other surface of the object 19.
In such a mounted state of the tee nut 31, the pawls 37 to 40 inhibit the tee nut 31 from rotating with respect to the object 19 while the flange portion 34 and the caulked part 34a hold the object 19 therebetween for preventing displacement of the tee nut 31 from the through hole 18. Therefore, the tee nut 31 is strongly fixed to the object 19 and semi-permanently kept in this fixed state.
The female screw forming part 15 or 35 of the aforementioned conventional tee nut 11 or 31 is threaded with a so-called bent tap, for example. FIGS. 17 and 18 show a bent tap 71 continuously threading a plurality of intermediate products 73 for tee nuts.
In this tapping step, the bent tap 71 is held in a hollow rotary spindle 70 for rotating at about 1000 to 3000 rpm. The plurality of intermediate products 73 for tee nuts are continuously fed along a chute 72, and a stopper 74 is driven to guide the intermediate products 73 one by one to an end of the bent tap 71 and a push rod 75 presses each intermediate product 73 against the bent tap 71 for starting the threading process. Since the bent tap 71 rotates in the direction of arrow Y with the rotary spindle 70, each threaded tee nut is moved in a direction Z along the bent tap 71 and pushed by the subsequent tee nut to be continuously moved after passing through a cutting edge portion of the bent tap 71 held by a tap cover 76 from both sides. Consequently, the tee nut reaching the other end of the bent tap 71 through the inner side of the rotary spindle 70 is discharged from an opening 70a of the rotary spindle 70. Thus, formation of a female screw or threading is completed to provide a desired tee nut.
The shaft portion 12 or 32 of each of the aforementioned conventional tee nuts 11 and 31 has a substantially constant outer diameter along the overall length thereof and the region excluding the predeterminate caulked part 14 or 34 has a substantially constant inner diameter along the overall length. Thus, a female screw having a threading groove shape agreeing with a standard such as JIS (Japanese Industrial Standard) is formed along the overall length of this region. The female screw of this type of tee nut can serve its function even if the shaft portion has a large length, so far as the length of the female screw having the groove shape agreeing with the standard is at least substantially identical to the root diameter thereof. For example, a hexagon nut provided with a female screw having a nominal diameter d in the range of 1.6 to 39 mm with a nominal height of at least 0.8 d (complete thread length of at least 0.6 d) is defined as "normal height nut" in relation to B1052 (corresponding to ISO 898-2: 1980 and ISO 898-6: 1988) of JIS.
When the structure of the aforementioned conventional tee nut 11 or 31 is applied to a tee nut having a relatively long shaft portion, therefore, the length of the female screw forming part 15 or 35 increases beyond necessity and hence additional working is required for the threading step with a bent tap, which deteriorates the productivity of manufacturing of the tee nut 11 or 31.
In the step of forming the female screw with the bent tap 71 shown in FIGS. 17 and 18, further, feeding of the threaded tee nut is hindered at a bent portion of the bent tap 71 if a hollow cylindrical portion of the intermediate product for the tee nut to be provided with the female screw has a long and thin inner periphery, which makes it impossible to form the female screw with the bent tap 71.